Inorganic Borate Functionalized {Ni6SiW9} Polyoxometalates: From Cluster to Cage Cluster

In aqueous solution systems, an all-inorganic Ni₆-added polyoxometalate (Ni₆AP) [Ni₆(μ₃–OH)₃(H₂O)₁₂(B-α-SiW₉O₃₄)]^1– ({Ni₆SiW₉}, here, {Ni₆} = Ni₆(μ₃–OH)₃(H₂O)₁₂) has been made via a hydrothermal method. By introducing inorganic boron sources, a B₃O₃(OH)₆ ({B₃}) cluster was precisely anchored onto {Ni₆} to form a [B₃O₃(OH)₆Ni₆(μ₃–OH)₂(H₂O)₈(B-α-SiW₉O₃₄)]^3– ({B₃Ni₆(OH)₂(H₂O)₈SiW₉}) through replacing four terminal water molecules and condensing with one hydroxyl of the {Ni₆} cluster. Subsequently, the addition of further boron sources facilitated the in situ growth of B–O clusters on the basis of {B₃Ni₆(OH)₂(H₂O)₈SiW₉}, which resulted in the successful synthesis of {[B₇O₈O_2/3(OH)₆Ni₆(μ₃–OH)₂(H₂O)₈(B-α-SiW₉O₃₄)]₃}^7– ({B₇Ni₆(OH)₂(H₂O)₈SiW₉}₃ or {B₂₁}@{Ni₆(OH)₂(H₂O)₈SiW₉}₃). It is the first all-inorganic cage cluster constructed by NiAPs and B–O clusters, which successfully realizes the inorganic borate functionalized POMs from a single cluster to a cage cluster. The central {B₇O₈O_2/3(OH)₆}₃ ({B₂₁}) cluster exhibits an uncommon cavity configuration. Meanwhile, we have conducted a discussion of the possible formation mechanism of B–O functionalized NiAPs. Furthermore, these three compounds efficiently accelerate the oxidative transformation of sulfides, and the number of B atoms in compounds relatively affects the selectivity of the products.